12.5-13V in rest shows a good battery. We can rule out the battery as a cause.

You've cleaned the contacts (including both contacts of the ground strap from the chassis to the battery and the connections to the starter itself), so the starter is suspect. You can measure if 12V is fed to the starter during starting. The starter can get dirty or worn out, usually a cleaning is sufficient to have a good working starter again, sometimes the brushes of the starter need replacing.

You can also measure if the glow plugs get 12V. Each of the glow plugs gets 12V, if you measure at the connection, it should read 12V during pre-glowing.

I know the starter is getting every bit of the 12 volts the battery has.

Last night my generator light came back on like it should for no reason.
I think I have an aftermarket lift pump. Anyway, it went out for a few minutes, then came back on like it should be. Nothing had changed to make it do that.

I tried turning the engine over last night after messing around on it with my brother. When I turned it over, it would crank a little slow like it has been, but occasionally turn over once or twice FAST like it used to, then slow down again.
My brother suspects a fuel filter is clogged.
I know that usually shows on the high end with these, but is that a possibility worth exploring?
Also, my fuel tank is down to about "0/4". Could I have sucked up bad fuel?

The fuel filter might be clogged, but that would not cause a slow starter. It can prevent starting (if it is clogged pretty badly). I don't think the W115 has a tank strainer (as the w123 has), so that will not get clogged. The pre filter can get clogged, as well as the main filter. A clogged prefilter is usually black. The main filter can also be inspected.

What makes you think you have an electric lift pump?

When you put the ignition in position 2, there will be a current from the battery, through the alternator light and through the alternator and to ground. This current is necessary to "charge" the alternator so that it can produce current. If the engine is running, the alternator will produce current, and the alternator light will go out, because there is current on both sides of the light.

If the light doesn't come on, there could be a bad ground connection or the alternator is worn-out. Or there could be a problem with the circuit of the oil, tank and temperature clock or the wiring. The light itself could also be on the way out.

If the fuel is low, it suck particles and other stuff that accumulates in the tank over the years. A dirty prefilter will show that.

Because it should ALWAYS be the first thing you check on a diesel engine which sat for a few years.

Remember the diesel cycle. "Suck, squeeze, bang, blow". "Squeeze" is the most important part. If your compression isn't right then you'll have starting problems, especially as the weather turns cooler. I'll bet that even when the car was running for you, you had to crank the starter for 15 seconds or longer. An engine in proper shape, using the glow plugs, should fire on about the third crank.

A sign your compression is getting weak is having to use the glow plugs every time you start it. You should only need to use the glow cycle the first time in the morning and after it sits for a few hours on a cool-ish day (below 80 degrees).

It could also explain the idle problems you had originally. Ideally you should have about 350 p.s.i. and minimal would be 270 p.s.i. .

The excessive use of the starter would certainly explain why your starter is getting weak.

I took some sandpaper to the connections on the starter today, and it seems to turn over just about as fast as it used to.
However, I can't always get my glow plugs to work, and when I do, the indicator doesnt get NEARLY as bright as it used to.

I plugged my block heater in for 2 hours and tried to start it... unsuccessfully.
That part makes me agree with you on the compression.

When this thing ran, it would take about 10 cranks(2-3 seconds) to start it running with the new glow plugs. The longest it ever took with glow plugs was on a colder day and it took about 5-6 seconds.
The car would start reliably within a few turns with no pre-glow once it was warm.

I got to wondering...
Would all this turning over, with some people giving it gas the whole time, have put a little much diesel in the cycinders as to cause problems?

Also, it should be noted that my oil is horribly dirty and slow(result of blowby-poor compression?). I jokingly referred to it as 'silly putty' the other day.
An exageration, but you get the point.
Also, my dipstick is marked "200" and something else on it. It's also a terrible fit in the car. I may have the wrong dipstick.
On the topic of oil, when I open the lid on the valve cover, should I see oil inside? I don't right now, but I was just curious.

Well, that sounds a lot more hopeful. Do the compression test to eliminate possible worn rings, cracked head, blown head gasket, etc.

Pull ALL of whatever you are removing to test the compression, i.e. if you testing through the injector holes, pull all the injectors before running the test. This eliminates the chance of a head gasket being bad and the cylinder you are testing being pressurized by the cylinder next to it.

You need to be sure your starting point is good (basic engine integrity) before you attempt to build on it. You don't build a house on a rickety foundation and attempt to correct it while framing the walls. Well, I don't anyway, I can't speak for most of the "McMansion" builders I've seen.

The engine was running and starting before, there is no reason why compression should fail so suddenly that the car isn't starting.

There is no harm in doing a compression test, but I don't think low compression is the cause for not starting. The time it took to start the engine don't sound unusual. Compression will decrease over time, because pistons and cylinders wear out, but not all of a sudden.

More likely there is a problem with the power supply to the glowers or the fuel supply. The electric lift pump isn't stock.

The glow plug indicator light isn't stock either, it was probably installed when the original glow plugs were installed. Originally there was a extra glow plug fitted on the dashboard behind the salt shaker, which started to glow red after 30 seconds or so. The upgrade to the newer glow plugs probably meant that a relay was installed with a indicator light. The fact that the light isn't glowing could mean the relay isn't working properly. Another reason might be a bad light or a bad connection to the light. You can try putting 12V directly to the glow plugs from the battery; do that for about 20 seconds and try starting the engine (an assistant is handy).

The fuel supply can be checked by opening the hard fuel lines to the injectors at the injectors. Just give it half a turn and try starting: diesel must squirt out. Do that for every cylinder.

Diesel in the cylinders isn't really a problem, you cannot flood it. Putting the accelerator pedal down during starting is not necessary and does not produce more fuel. When you pull on the gorilla knob during starting, you'll pull the lever on the pump in the starting position, that means that more than the normal maximum amount of fuel is injected. The position of the accelerator pedal doesn't change that.

The starter seems to be OK now, but if you want to clean it, you'll have to remove it from the car. The starter is connected to the engine with a couple of hex bolts and a bracket.

You might want to take a look at this short film, so you'll know how it works and what the parts are:

There are many descriptions of taking apart the starter on the big ol' internet.

The three screws of the solenoid are usually stuck. Proceed with care and WD40 here.

Remove the cap on the back, remove the locking ring on the axle

Unscrew the two long screws, visible in the picture below:

The bridge with the brushes looks like this:

Two of the brushes are connected to the starter housing, but you can remove them from the bridge if you remove the springs from the back. These springs are likely to spring away to an unknown destination, so take care.

The brushes should have some length, as is shown in the picture. Clean where the brushes connect to the rotating coil, shown on the left end in the picture below:

Use some fine oil when you put the thing together again. Don't oil the solenoid but clean and degrease it thoroughly.

Oil inside diesel engines will get dirty. You probably want to replace it. MB stated an intervat of 3000 miles, but that was with the 1970s oils. With modern oils you probably get away with 5000â€“7000 miles. It does not prevent starting, unless it is very thick.

You should not see oil when you remove the oil cap on top of the valve cover. Oil will have drained to the bottom, in the oil pan.